In a vehicle, a transmission is interposed between an internal combustion engine and the driving wheels. This transmission changes the driving force applied to and the speed of the driving wheels to meet the travel condition of the vehicle according to a wide range, in order to optimize the performance of the internal combustion engine. One known continuously variable transmission includes a pulley having a fixed pulley part fixed to a shaft and a movable pulley part axially movably supported on the shaft separate from the fixed pulley part, the width of a groove formed between the pulley parts being increased and decreased by oil pressure control to increase and decrease the rotating radius of a power transfer belt wound on the pulley, thereby changing the belt ratio. Continuously variable transmissions of this type have been disclosed for instance in Japanese Patent Laid Open Publication Nos. Sho 186656/1982, Sho 43249/1984, Sho 77159/1984 and Sho 233256/1986.
The continuously variable transmissions have an oil pressure clutch for controlling the power transmission using oil pressure. This oil pressure clutch is controlled differently in various types of control modes, based on signals representing the number of revolutions of the engine, the throttle valve position of a carburetor, or the like.
In the conventional belt ratio controller for a continuously variable transmission, the belt ratio, if the speed of a vehicle is .ltoreq.5 km/h (including a stop time), is controlled to a full low condition by open loop control using an output duty ratio value of 0. After starting, when the speed of the vehicle is raised to bring the clutch slip to substantially zero, the oil pressure clutch is engaged to start to control the number of revolutions of the engine to a target value determined from a shift selector position, the opening of the throttle and the speed of the vehicle. The belt ratio is controlled to be between the full low and overdrive conditions when the number of revolutions of the engine starts to be controlled, thereby changing the speed.
In a case where the vehicle is started on the snow in a cold place or on a frozen road (especially where the vehicle is started on a slope), namely when a friction coefficient on a road surface is small, a transfer torque is large in the full low condition of the belt ratio, so that when abruptly stepping on an accelerator pedal, the wheels slip and cause the vehicle inconveniently to be incapable of starting to move.
When the slipping state of the wheels is continued for a long time, the control arrangement decides to adjust the speed of the vehicle and the oil clutch is engaged, so that the slip of the wheels is further continued and causes friction heat therefrom to dissolve the snow or ice and the vehicle to inconveniently be permanently incapable of being started.
A purpose of the present invention is to provide a belt ratio controller for a continuously variable transmission which controls the belt ratio to a target belt ratio value set by the opening of the throttle at the time of starting on a road having a small road surface friction coefficient in order to avoid the aforementioned inconveniences, thereby transferring a low engine torque preventing the wheels from slipping, to facilitate easy starting of the vehicle.